The present invention relates to a communication apparatus.
One modulation technique currently in general use is I/Q modulation. With this technique, a modulation signal is treated as a complex amplitude with a real number portion corresponding to an in-phase component and an imaginary number portion corresponding to a quadrature component. In other words, the points in an X-Y coordinate system are used as data, and the data is expressed as an X-Y signal which is multiplied by the carrier frequency.
Recently, another method called polar modulation has come into use. According to polar modulation, modulation is performed by expressing the data as an amplitude and an angle. A frequency synthesizer then performs modulation in the angular direction using frequency modulation. Meanwhile, the modulation in the amplitude direction is used to control a gain of a power amplifier (hereinafter “PA”).
There are two well-known methods of amplitude modulation in polar modulation. In one method, source amplitude modulation data is converted to an analog controlling signal via a digital-to-analog converter (hereinafter referred to as a “DAC”) and a low-pass filter (hereinafter referred to as an “LPF”), and the analog controlling signal is fed to the PA to change the gain. This method has an advantage in that the effects of quantization noise included in the source data can be eliminated, but requires that the DAC and the LPF are provided. Moreover, the delays generated by the DAC and the LPF have to be corrected. It is further required that the PA has a linear gain with respect to the supplied amplitude data (analog controlling signal).
In the other method, the amplitude modulation data is not passed through the DAC and the LPF, and is instead provided in unmodified form as a digital value to a digitally controlled power amplifier (hereinafter DPA) for amplitude control. However, the dynamic range of the DPA is smaller than the dynamic range of the amplitude modulation data. This method has a further problem in that there is no function for removing the quantization noise, and so all the noise appears in the output.
To solve such problems a configuration has been proposed (see, for instance, Robert Bogdan Staszewski et. al, “All-Digital PLL and Transmitter for Mobile Phones” the Journal of Solid-State Circuits, December 2005, vol. 40, No. 12, 2469) in which a delta-sigma (ΔΣ) modulator or the like is used to perform oversampling at a frequency higher than a clock used to produce the amplitude modulation data, expand the dynamic range, spread the quantization noise to a high-frequency side, and improve the resolution of the original signal.
The ΔΣ modulator has a construction which has an offset. In modulation schemes in which the data points are located on the circumference of a circle such as PSK (Phase Shift Keying), small amplitude information does not have a large meaning, and so the offset of the ΔΣ modulator is not a problem.
However, in modulation schemes which include data points when the amplitude such as QAM (Quadrature AM) is small, the problem arises of not being able to correctly express the data due to the offset.